The present invention relates generally to packages for housing electronic circuit components, and more particularly to packages which are adapted to be surface mounted on a printed circuit board or the like without having to manipulate the conducting leads.
In establishing electronic circuits for RF, microwave and other applications, it has been desirable to employ electronic component packages which can be surface mounted to a printed circuit board (PCB), or other substrate. The surface mounting of the package is useful in supporting the package on the printed circuit board, but more importantly provides the advantages of a direct electrical contact or efficient heatsink. The present invention is directed to surface mounted packages which impart such advantages, though the present invention is applicable to packages which are not surface mounted and/or do not include bases adapted to provide a direct electrical contact or a heatsink.
In any type of package, the conducting or transmission leads of the surface mount package must be constructed and arranged so that the electronic components housed in the package can be connected to the other electronic components on the printed circuit board. Such transmission leads include the plug-in type --that is, disposed in holes in a printed circuit board, and subsequently soldered therein so that the connection to the other components on the printed circuit board is effected. This requires access to both sides of the printed circuit board. Thus, the more desirable type of package is the "surface mount" type in which the transmission leads are constructed and arranged to be in the same plane as the resting surface of the base and suitable for "pick and place" assembly where in a large-scale automated assembly line a robot can easily position the package for connection to the printed circuit board without having to insert the transmission leads into holes in the printed circuit board. This requires that the transmission leads, be in contact with the printed circuit board, and, with regard to surface mounted packages, the bottom surface of the package would also be in contact with the printed circuit board.
Since conventional packages, such as the so called "flat-pack", generally designated as A in FIG. 1, include elongate transmission leads which extend through the peripheral walls of the package in a generally horizontal manner, the construction of the package and the printed circuit board had to be reconciled so that the leads would contact the board, and in the case of a surface mounted package, the bottom surface of the package also contacts the board. In many cases, an opening is provided in the printed circuit board so that the bottom surface of the flat-pack hangs in the opening when the transmission leads are resting on the printed circuit board. This adds tremendous costs to the manufacture of the printed circuit board and limited the size of the package which could be used on the printed circuit board because of the mechanical constraints of securing a large package to a printed circuit board so that the connection of the package and its leads to the board will not fail under vibration or shock. Also, a package arranged in this manner can not dissipate heat as efficiently as surface mount packages. Further, this arrangement is susceptible to shorting out by any excess solder contacting the base of the package when soldering the transmission leads to the printed circuit board.
In many instances, the preferable method of mounting a flat-pack of the type shown in FIG. 1 is to manually bend the transmission lead C in two or more places so that the end portion of the transmission lead rests on the printed circuit board, and in the case of surface mounted packages, the bottom surface of the surface mounted packages, the bottom surface of the package itself also rests on the board. Thus, the transmission lead C shown in FIGS. 1 and 2 must be manipulated so that the bottom surface of the end portion is flush with the bottom surface of the package itself. Such manipulation is typically accomplished with a custom tool which is expensive and quite difficult to operate properly. The bent transmission lead E, shown in dashed formation in FIG. 2, illustrates the manner in which the lead and the package should rest on the printed circuit board. U.S. Pat. No. 3,061,762 exemplifies this bending technique for preparing a plug-in or surface mount package for mounting on a printed circuit board.
Few have appreciated the shortcomings of this latter technique which requires the intricate and time-consuming manual manipulation of the transmission leads C of the flat-pack A in FIG. 1. These flat-packs are made in conventional sizes and sold with the transmission leads C in the horizontal position. Further, since these standard packages are used in a variety of applications, the transmission leads C are of considerable length. It is left to the manufacturer of the electronic circuit in which the standard package will be used to not only bend the transmission leads C, but to also ensure that the resulting length thereof is appropriate for the particular printed circuit board. The result is that a large portion of real estate of the printed circuit board is unnecessarily utilized. Referring to FIG. 1, it is noteworthy that the bent transmission lead E is a considerable distance from the peripheral wall of the base B of the flat-pack A. This waste of real estate on a printed circuit board is quite expensive and also requires a larger amount of space to mount the printed circuit board.
Moreover, the considerable length of the transmission lead C required for manual bending is highly undesirable in high frequency applications. Indeed, the bends in the transmission lead E (in dashed formation) are also undesirable in high frequency applications since this amounts to discontinuities. Moreover, it is difficult to bend the plural leads of one package so that they are of the same dimensions with one another or so that they are of the same dimensions as the leads on another package. At high frequencies, this lack of "repeatability" from unit to unit adds to the discontinuities. The long length of the leads and the discontinuity at the bent portions results in losses and mismatches between the package and the other components on the printed circuit board. Thus, antithetical to this generally accepted technique, it is desirable to maintain a lead length which is as short as possible and to avoid manual bending of the leads. Also, the bending of the transmission leads C can result in cracks in the metalization at the corners of the bends where the shear forces are greatest.
Perhaps the greatest disadvantage of this generally accepted technique is the effect on the integrity of the glass-to-metal seal D provided around the transmission lead C to insulate the transmission lead C from the base B and the hermetically seal the opening in which the transmission lead C is disposed. The manual manipulation of the transmission leads C can easily result in hairline fractures in the glass-to-metal seal. Quite often, the damage to the glass-to-metal seal D is not appreciated until there is a failure, and the package must be replaced--by another package which has leads to be bent.
Beyond the above techniques, there is little done in the industry to provide an installation technique or an inexpensive and effective surface mount package which is suitable for most applications, including high frequency applications. For instance, U.S. Pat. No. 4,644,096 discloses a modified TO-8 package in which an L-shaped conducting lead extends through a hole in the base beyond the bottom surface of the base. A glass-to-metal seal is provided around the conducting lead in the opening of the base. A circular copper spacer is specifically provide for connection to the base interiorly of the conducting leads such that the bottom surface of the conducting lead is flush with the bottom surface of the copper spacer. Grounding holes are provided in the base to solder the copper spacer to the base and establish electrical contact therebetween. An epoxy secures the L-shaped lead in the area between the base, glass-to-metal seal and copper spacer. Those skilled in the art will recognize that the intricate structure of the package disclosed in this patent is difficult to manufacture and cost prohibitive. Moreover, the proximity of the L-shaped conducting leads and the copper spacer renders this package highly susceptible to shorting. It is believed that in view of these reasons, the package disclosed in U.S. Pat. No. 4,644,096 has never been successfully commercialized.
U.S. Pat. No. 3,833,753 discloses a package which includes a lead structure made of two leads soldered together and a ceramic wafer element which is metallized on one side. This construction is then connected to the copper base of the package. As with the previously described package, the intricate construction of this package is expensive and difficult to manufacture.
It is thus apparent that an improved electronic component package which overcomes the above shortcomings and disadvantages is warranted. Such an electronic component package would be easy to manufacture, easily installable on a printed circuit board, have substantially no size constraints, could be used for virtually any application, and can replace existing conventional packages without the need to restructure the printed circuit board.